[CodeGen] Ignore return sext/zext attributes of unused results for tail calls

If the caller's return type does not have a zeroext attribute but the
callee does a tail call zeroext, we won't consider the tail call during
CodeGenPrepare because the attributes don't match.

However, if the result of the tail call has no uses, it makes sense to
drop the sext/zext attributes.

Differential Revision: https://reviews.llvm.org/D56486

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350753 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/CodeGen/Analysis.cpp b/lib/CodeGen/Analysis.cpp
index 27dce7fd7b79981e882e1d56adfedd5fd7994256..797f05ee5cf348fdae6b9801fc9696b29ebdd798 100644 (file)
--- a/lib/CodeGen/Analysis.cpp
+++ b/lib/CodeGen/Analysis.cpp
@@ -546,6 +546,21 @@ bool llvm::attributesPermitTailCall(const Function *F, const Instruction *I,
     CalleeAttrs.removeAttribute(Attribute::SExt);
   }
 
+  // Drop sext and zext return attributes if the result is not used.
+  // This enables tail calls for code like:
+  //
+  // define void @caller() {
+  // entry:
+  //   %unused_result = tail call zeroext i1 @callee()
+  //   br label %retlabel
+  // retlabel:
+  //   ret void
+  // }
+  if (I->use_empty()) {
+    CalleeAttrs.removeAttribute(Attribute::SExt);
+    CalleeAttrs.removeAttribute(Attribute::ZExt);
+  }
+
   // If they're still different, there's some facet we don't understand
   // (currently only "inreg", but in future who knows). It may be OK but the
   // only safe option is to reject the tail call.

diff --git a/test/CodeGen/AArch64/tail-call-unused-zext.ll b/test/CodeGen/AArch64/tail-call-unused-zext.ll
new file mode 100644 (file)
index 0000000..1617d13
--- /dev/null
+++ b/test/CodeGen/AArch64/tail-call-unused-zext.ll
@@ -0,0 +1,36 @@
+; RUN: llc -mtriple=arm64--- -stop-after=expand-isel-pseudos -o - %s | FileCheck %s
+
+; Check that we ignore the zeroext attribute on the return type of the tail
+; call, since the return value is unused. This happens during CodeGenPrepare in
+; dupRetToEnableTailCallOpts, which calls attributesPermitTailCall to check if
+; the attributes of the caller and the callee match.
+
+declare zeroext i1 @zcallee()
+define void @zcaller() {
+; CHECK-LABEL: name: zcaller
+entry:
+  br i1 undef, label %calllabel, label %retlabel
+calllabel:
+; CHECK: bb.1.calllabel:
+; CHECK-NOT: BL @zcallee
+; CHECK-NEXT: TCRETURNdi @zcallee
+  %unused_result = tail call zeroext i1 @zcallee()
+  br label %retlabel
+retlabel:
+  ret void
+}
+
+declare signext i1 @scallee()
+define void @scaller() {
+; CHECK-LABEL: name: scaller
+entry:
+  br i1 undef, label %calllabel, label %retlabel
+calllabel:
+; CHECK: bb.1.calllabel:
+; CHECK-NOT: BL @scallee
+; CHECK-NEXT: TCRETURNdi @scallee
+  %unused_result = tail call signext i1 @scallee()
+  br label %retlabel
+retlabel:
+  ret void
+}